1. Field of the Invention
The present invention relates to a pneumatic tire comprising a circumferential belt layer made of a rubber-coated cord extending along a tire equatorial plane and to a method of forming the circumferential belt layer.
2. Description of Related Art
In recent years a tire is flattened due to demand for a faster vehicle with a low-floor. In connection with this, a tread portion is further expanded in the radial direction, that is to say, the amount of the radial expansion (diameter growth) of the tread portion is further increased when the tire is inflated with a normal internal pressure. The increase in the radial expansion of the tread portion causes enhancement of stress concentration in belt ends and degradation of durability performance in the belt ends, in turn, causes early separation at the belt ends.
In other words, in a tire with a small flatness ratio, since the increase in the radial expansion of the tread portion, especially the portion near the shoulder portion when the tire is inflated with an internal pressure becomes a problem, a technique to restrain radial expansion by using a circumferential belt layer in which reinforcing elements extend in the circumferential direction of a tire is necessary.
As the technique to restrain radial expansion by the circumferential belt layer, JP2-208101 discloses a tire comprising a carcass, at least two inclined belt layers disposed on the carcass and at least one circumferential belt layer disposed on a radially inner side of the inclined belt layers, wherein each of the inclined belt layers has a plurality of reinforcing cords extending with an inclination angle of 10° to 40° with respect to the tire equatorial plane and intersecting with each other across the tire equatorial plane and the circumferential belt layer is made of a strip having a plurality of reinforcing cords which are shaped in a wave or zigzag pattern and extending along the tire equatorial plane as a whole.
An example of a conventional method of forming a circumferential belt layer will be described with reference to FIG. 1 showing a development plane view of a drum around which the circumferential belt layer is wound. The circumferential belt layers are formed by spirally winding a strip along the tire equatorial plane, which the strip is made of at least one rubber-coated cord.
Specifically, as shown in FIG. 1 (a), the strip S is wound in the direction shown by the arrow on the drum D from a position A adjacent to the tire equatorial plane CL in the inner side in the tire width direction to a position B′. While the strip is wound on the drum D from the position A to the position B′ for the length of the drum D, the strip is shifted for the width of the strip S in the outer side in the tire width direction. Successively, the strip S is wound on the drum D from the position B (the positions B′ and B indicate the same position) to the position C′.
Next, as shown in FIG. 1 (b), the strip S is wound in the direction shown by the arrow on the drum D in parallel to the tire equatorial plane CL from the position C to the position C′ to form the edge E in the tire width direction of the circumferential belt layer.
Next as shown in FIG. 1 (c), the strip S is wound in the direction shown by the arrow on the drum D from the position C to the position B′ and successively, the strip S is wound on the drum D from the position B to the position A′. The subsequent winding of the strip S is not illustrated but the strip is continued to be spirally wound on the drum D.
In FIG. 1 the strip S adjacent to each other slightly keeps away for illustrative purposes but the strip S does not keep away in practice.
In the case where the circumferential belt layer is formed by spirally winding the strip S as mentioned above, if the winding of the strip S is started from the outer side in the tire width direction, a winding start edge and a winding end edge, which are cutting edges of the cord are to be exposed. In this case, separation caused by the winding start edge and the winding end edge may be generated.
Therefore, it is preferable that the winding of the strip S begins in the neighborhood of the tire equatorial plane CL, which is the inner side in the tire width direction, the winding reaches one outer edge and then turned back to the inner side, the winding reaches the other outer edge and then turned back to the inner side and finally the winding is finished near the beginning location of the winding.
In this case, as shown in FIG. 1 (b), the circumferential belt layer is wound for the length of the drum D in parallel to the tire equatorial plane CL in the edge E in the tire width direction of the circumferential belt layer. If the strip S is turned back to the inner side without winding the strip S for the length of the drum D in parallel to the tire equatorial plane CL in the edge E, not only hardness in the circumferential direction in the edge of the circumferential belt layer is insufficient to support an internal pressure but also the cord of the edge is broken down due to fatigue since stress in the edge is increased when the tire contacts the ground under load.